Interim Standard IS-95-A (IS-95) has been adopted by the Telecommunications Industry Association for implementing CDMA in a Personal Communications Services (PCS) or cellular system. In either the PCS or cellular system, a mobile station, such as a wireless communication device, communicates with any one or more of a plurality of base stations dispersed over a geographic area. In a system that employs CDMA technology, the down-link communication between the base stations and the mobile stations can take place over a Pilot Channel, Forward Traffic Channels, and a Paging Channel, among others.
On the Pilot Channel, each base station continuously transmits a pilot signal having the same spreading code but with a different phase offset. A mobile station can distinguish the pilot signals from one another by the applied phase offset, which allows the mobile station to identify the base station transmitting the pilot signal. Furthermore, a mobile station can measure the signal-to-noise of each pilot signal, which indicates the pilot's relative signal strength.
IS-95 specifies four sets of pilots collectively referred to as the Pilot Set--the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. The Active Set are pilots associated with Forward Traffic Channels assigned to the mobile station for demodulating calls. That is, when in a "call," the mobile station demodulates the Forward Traffic Channels of the pilots of the Active Set. The Candidate Set are pilots that have been received by the mobile station with sufficient strength to indicate that the associated Forward Traffic Channels could be successfully demodulated, but are not currently in the Active Set. The Neighbor Set are pilots that are not currently in the Active Set or Candidate Set but are likely future candidates for handoff. The Neighbor Set normally corresponds to pilots that are in close geographical areas to the mobile station. The Remaining Set are all possible pilots in the current system on the current CDMA frequency assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the Active Set.
As the mobile station moves from a region covered by one pilot to another region covered by another pilot, the relative strength of the pilot signals as received by the mobile station will change, and it will be desirable to hand off to another pilot. According to IS-95, the mobile station--whether in an idle state or in a call--is required to assist in the handoff process by measuring the strength of pilot signals.
When in the idle state, the mobile station continuously searches for the strongest pilot signal in the Active Set, Neighbor Set, and Remaining Set. If the mobile station determines that one of the Neighbor Set or Remaining Set pilot signals is sufficiently stronger than the pilot of the Active Set, the stronger pilot is placed in the Active Set, and an idle handoff is thus made to the stronger pilot.
When in a call, the procedure for handoff is more complex, and involves the Candidate Set and the system's infrastructure. The mobile station promotes stronger pilots from the Neighbor Set or the Remaining Set to the Candidate Set, and then notifies the infrastructure of the new Candidate Set via a Pilot Strength Measurement Message over the Paging Channel. After receipt and evaluation of the Pilot Strength Measurement Message, the infrastructure may then promote some of the pilots of the new Candidate Set to the Active Set, thus creating a new Active Set. The infrastructure will subsequently notify the mobile station of the new Active Set via a Handoff Direction Message over the Traffic Channel; and, the mobile station will update its Active Set, Candidate Set, and Neighbor Set in accordance with the Handoff Direction Message. The mobile station will then demodulate the call using the pilots of the Active Set, to which the call is handed off. This process of updating the Pilot Set is commonly referred to as "pilot set maintenance."
The Paging Channel is used not only for transmitting control information relating to pilot set maintenance, but also for notifying the mobile station of an incoming call by way of a page. Because pages to a particular mobile station may occur infrequently, IS-95-A provides for a slotted mode feature, which allows a mobile station to operate in a reduced power mode, thus conserving battery power of the mobile station.
The Paging Channel is divided into 80 millisecond (ms) intervals called paging channel slots, and each mobile station operating in the slotted mode is assigned a specific slot of a periodic slot cycle in which they monitor the Paging Channel. For example, a slot cycle with a period of 2.56 seconds has 32 slots of 80 ms each. Because the mobile station needs only to monitor the Paging Channel during its assigned slot, at all other times of the slot cycle the mobile station can "sleep," that is, go into the reduced power mode by turning off all functions not necessary to "wake up "the mobile station in time to receive the assigned slot.
Because the mobile station is not able to receive and measure pilot signal strengths while it sleeps, idle handoffs and normal pilot set maintenance are not occurring. While asleep, the mobile station may move into another region, and, consequently, the Active Set before going to sleep will not specify the strongest pilot in the new location. Consequently, when the mobile station awakes, it will not be monitoring the Paging Channel transmitted from the pilot with the strongest signal.
A conventional way for acquiring the strongest pilot for monitoring the Paging Channel is for the mobile station to wake up early enough to search for the strongest pilot of the Active Set and the Neighbor Set. Each pilot is scanned and the signal strength measured. The pilot that has the strongest received signal that is sufficiently greater than the measured signal strength of the pilot of the Active Set is then assigned to the Active Set, and a handoff to the stronger pilot occurs before the assigned slot for monitoring the Paging Channel.
A scan of the pilots of the Active Set and the Neighbor Set prior to the assigned slot has the disadvantage of consuming a portion of the slot cycle that the mobile station could be asleep. For example, observations of the conventional mobile station revealed that, for a slot cycle of 1.28 seconds and a Neighbor Set of 10 pilots, the mobile station is awake 67 percent of the 1.28-second slot cycle.
A need therefore exists for a wireless communication device and a method of handing off that reduces the amount of time that the mobile station is awake during the slot cycle.